With faster ICs, a variation of electric current working therethrough must be great, and a source voltage is varied by high-speed switching so that they are apt to be badly influenced by noises. Further, an electric power consumption of ICs and driving voltage thereof are reduced, so the driving voltage is highly varied by a slight variation of the source voltage; namely ICs are apt to be badly influenced by a voltage variation. The above described problems disturb faster ICs, thus a decoupling capacitor is provided between a source line and an earth line of a semiconductor element to solve the problems.
In case of providing the decoupling capacitor, an IC is highly densified and number of working elements in the IC must be increased, so a high-capacity capacitor must be provided and located close to the working elements so as to shorten signal paths and reduce inductance to high-speed signals. Thus, conventionally, the capacitor is provided on a semiconductor element mounting surface of a substrate or on the opposite surface thereof and immediately under the semiconductor element so as to locate the capacitor close to the semiconductor element.
Note that, substrates for semiconductor devices, in each of which a decoupling capacitor is formed instead of the capacitor separated from the substrate, were invented. For example, the decoupling capacitor is formed by a build-up process when a cable layer is formed on the substrate (see Patent Document 1), and the decoupling capacitor and a cable layer are formed in a circuit board and used as an interposer (see Patent Document 2).
Patent Document 1: Japanese Patent Laid-open Gazette No. 2003-133507; and
Patent Document 2: Japanese Patent Laid-open Gazette No. 2001-250885